HARMONY IN MUSIC. 61 



that the human ear is affected by vibrations of the air, within 

 certain degrees of rapidity viz. from about 20 to about 32,000 

 in a second and that the sensation of musical tone arises from 

 this affection. 



That the sensation thus excited is a sensation of musical 

 tone does not depend in any way upon the peculiar manner in 

 which the air is agitated, but solely on the peculiar powers of 

 sensation possessed by our ears and auditory nerves. I re- 

 marked, a little while ago, that when the tones are loud the 

 agitation of the air is perceptible to the skin. In this way 

 deaf nrntes can perceive the motion of the air which we call 

 sound. But they do not hear, that is, they have no sensation of 

 tone in the ear. They feel the motion by the nerves of the skin, 

 producing that peculiar description of sensation called whirring. 

 The limits of the rapidity of vibration within which the ear 

 feels an agitation of the air to be sound, depend also wholly 

 upon the peculiar constitution of the ear. 



When the siren is turned slowly, and hence the puffs of 

 air succeed each other slowly, you hear no musical sound. By 

 the continually increasing rapidity of its revolution, no essential 

 change is produced in the kind of vibration of the air. Nothing 

 new happens externally to the ear. The only new result is the 

 sensation experienced by the ear, which then for the first time 

 begins to be affected by the agitation of the air. Hence the 

 more rapid vibrations receive a new name, and are called Sound. 

 If you admire paradoxes, you may say that aerial vibrations do 

 not become sound until they fall upon a hearing ear. 



I must now describe the propagation of sound through the 

 atmosphere. The motion of a mass of air through which a 

 tone passes belongs to the so-called wave-motions a class of 

 motions of great importance in physics. Light, as well as 

 sound, is one of these motions. 



The name is derived from the analogy of waves on the sur- 

 face of water, and these will best illustrate the peculiarity of 

 this description of motion. 



When a point in a surface of still water is agitated as by 

 throwing in a stone the motion thus caused is propagated in 



